1. Field of the Invention
This invention relates to a pulse tube refrigerator, and more particularly to a structure of a four valve type pulse tube refrigerator.
2. Discussion of the Background
A conventional four valve type pulse tube refrigerator is shown in FIG. 11. In the drawing, a four valve type pulse tube refrigerator 201 includes a compressor 1, a first high pressure on-off valve 4 connected with an outlet port 1a through a first level high pressure passage 2, a first low pressure on-off valve 5 connected with an inlet port 1b through a first level low pressure passage 3, a regenerator 8 having a low temperature end 8a and a high temperature end 8b, the regenerator being connected with the first high pressure on-off valve 4 through a regenerator side high pressure passage 6 at the high temperature end 8b of the regenerator 8 and being connected with the first low pressure on-off valve 5 through a regenerator side low pressure passage 7, cold head 9 connected with the low temperature end 8a of the regenerator 8, a pulse tube 10 having a low temperature end 10a and high temperature end 10b and connected with the cold head 9 at the low temperature end 10b thereof, a second high pressure on-off valve 11 connected with a radiator 15 attached to the high temperature end 10b through a pulse tube side high pressure passage 16, a second low pressure on-off valve 12 connected with the radiator 15 through a pulse tube side low pressure passage 17, a second level high pressure passage 13 connected with the first level high pressure passage 2 at one end and connected with the second high pressure on-off valve 11 at the other end, and a second level low pressure passage 14 connected with the first level low pressure passage 3 at one end and connected with the second low pressure on-off valve 12 at the other end. The space defined by the four valves (first high pressure on-off valve 4, second high pressure on-off valve 11, first low pressure on-off valve 5 and second low pressure on-off valve 12) and the compressor 1 operates as a working space (or an operating space) of the pulse tube refrigerator 201.
FIG. 12 illustrates valve on-off operations of the first high pressure on-off valve 4, the second high pressure on-off valve 11, the first low pressure on-off pressure valve 5 and the second low pressure on-off valve 12 in accordance with the operation of the pulse tube refrigerator 201 of FIG. 11. In the drawing, bold lines show the valve-on conditions and fine lines show the valve-off conditions of the valves. As shown in FIG. 12, the four valves, i.e., first high pressure on-off valve 4, second high pressure on-off valve 11, first low pressure on-off valve 5, and second low pressure on-off valve 12 are turned on in sequence. By sequentially turning on the four valves, the phase between the pressure changes of the operational gas, and movements (displacements) thereof, is controlled to generate a refrigeration output within the pulse tube 10 and extract a low temperature at the cold head 9. (see FIG. 11)
The above explained conventional four valve type pulse tube refrigerator and other conventional four valve type refrigerators have basic problems which cannot be solved, due to their operating principle. The problems are the low refrigeration efficiency and the lack of operating stability. The main cause of the low refrigeration efficiency is the operational loss caused by the returning of the operational gas from the operating space into the compressor. That is, a portion of the operational gas which has entered the operating space from the compressor 1 through the second high pressure on-off valve 4 during the operation of pulse tube refrigerator returns to the compressor 1 again from the second level low pressure passage 14 through the second low pressure on-off valve 12. The compressor 1 therefore has to work extra due to the unnecessary flow of the returning gas thereinto. This extra work of the compressor causes a low efficiency of the refrigerator.
The main cause of the lack of stability is the generation of a one way flow gas whose amount exceeds a fixed amount defined by the cycle within the pulse tube 10 due to the operational gas flow (in and out) from the compressor 1 in both directions from regenerator 8 side and pulse tube 10 side. (This one way flow gas may be defined as a circulation flow gas if the flow passages are included.)
The one way or circulation flow gas runs from the compressor 1 into the operating space through the first level high pressure passage 2 and the first high pressure on-off valve 4 and returns to the compressor 1 through the second low pressure on-off valve 12 and the second level low pressure passage 14. In another flow route, the operating gas runs from the compressor 1 into the operating space through the second level high pressure passage 13 and the second high pressure on-off valve 11 and returns to the compressor 1 through the first low pressure on-off valve 5 and the first level low pressure passage 3. Such a flow route may be variable depending on the operational conditions of the refrigerator 201. The above extra one way or circulation flow causes instability of the operation of the refrigerator.